Method for production of particulate metal



United States Patent US. Cl. 75-.5 10 Claims ABSTRACT OF THE DISCLOSUREA composite body having a porous matrix structure of crushable, friablematerial and having solid material contained within the pores of thematrix structure is provided. The composite body is crushed to freeparticulated metal from the matrix structure, thereby providing amixture of particulate metal and matrix material which has been reducedto a predetermined degree of fineness.

This invention relates to the preparation of particulate metals, andmore particularly to the production of particulate light metals such asmagnesium, aluminum and alloys thereof.

Methods well known for the production of particulate metals include, forexample, the atomization of molten metal by means of the impingement ofa stream of molten metal upon a rapidly rotating disc (Br. 510,320), orby the impingement of a high velocity gas jet upon a thin stream ofmolten metal (US. 2,676,359), by means of shot towers, by the shockchilling of gaseous metals, and by the mechanical reduction of masses ofsolid metal by machining, ball-milling, and other mechanical means wellknown in the art.

These art processes sulfer from one or more of many disadvantagesincluding: (1) the use of complex and expensive equipment and controls,(2) the hazardous handling of molten or gaseous metal at hightemperatures and the attendant corrosion problems involved in suchhandling, (3) fire and ignition hazards either inherent in the handlingof liquid or gaseous metals or resulting from the generation of largequantities of heat in the mechanical reduction of solid metal to afinely divided state, (4) the necessity of accomplishing the desiredsize reduction and rapidly cooling the finely divided metal whilemaintaining a carefully controlled inert atmosphere, (5) high energyrequirements, and (6) low yields of comminuted metals of the desiredparticle size.

It is a principal object of the present invention to provide aninexpensive method for the production of particulate metals which iseasily carried out with simple equipment. Another object is to provide amethod which requires a minimum of energy to reduce masses of solidmetal to a finely divided state. A further object is to provide a methodwherein fire hazards are minimized, particularly with light metals suchas magnesium. Other objects and advantages will become apparenthereinafter.

In carrying out the process of the present invention a composite bodyhaving a porous matrix structure of crushable, friable material andhaving solid metal contained within the pores of the matrix structure isprovided. The composite body is crushed to free particulated metal fromthe matrix structure, thereby providing a mixture of particulate metaland matrix material which has been reduced to a predetermined degree offineness. The particulate metal may, if desired, be separated andrecovered from said mixture by a variety of means well known to oneskilled in the art such as, for example, by screening, flotation,cyclone separation and the like.

One unexpected advantage of the present invention is that the presentnovel process provides for the direct prep- 3,482,963 Patented Dec. 9,1969 aration of useful mixtures of the particulate metal and the matrixmaterial. For example, where the matrix material is a reducible fuelsuch as coke and the metal is magnesium, the resulting particulatemixtures are suitable for use as components of incendiary bombs,pyrotechnics, and explosives.

The present invention is usually carried out by providing a porous,crushable medium having interconnected pores of predetermined size, saidmedium being thermally stable and not detrimentally attacked at processtemperatures, preheating said porous medium, introducing molten metalinto the pores of said preheated porous medium, and cooling the thusimpregnated porous medium to solidify the metal within said pores. Thecooled composite body is then subjected to sufiicient crushing action byemploying any suitable means such as, for example, a ball-mill,hammer-mill, crushing rolls, or the like to particulate the solidifiedmetal and to reduce the matrix material to a predetermined degree offineness. The control of the crushing operation and the selection of theequipment by which it is carried out will depend upon several factors,It is usually desirable to reduce the particulate metal to a particulatesize corresponding substantially to the predetermined pore size of theporous medium, i.e., to shear or fracture the metal which was containedwithin the innercon'necting passages between the pores of the matrixmaterial. The friability of the matrix material, the particle size towhich it is to be reduced and like considerations will determine theextent and nature of the crushing operation. If it is desired toseparate and recover the particulate metal from the matrix materialother factors to be considered or controlled to achieve optimumseparation, as will be well understood by one skilled in the art, willinclude the relative volumes, densities, and average particle sizes ofthe particulate metal and the matrix material.

In a preferred embodiment, the metal to be particulated is introducedinto porous media by vacuum casting, i.e. utilizing said porous media asmold forms. Conventional vacuum casting techniques Well known in the artcan be employed. For example, the integral porous bodies to beimpregnated and a sufiicient quantity of the solid metal to be cast are:placed in a suitable vessel. The vessel is closed, evacuated, andheated to melt the metal and immerse the porous bodies in molten metal.The vessel is then back-filled with an inert gas to relieve the lowpressure and substantially completely impregnate the porous bodies.

However, in a more preferred vacuum casting technique, the integralporous mold bodies of suitable refractory material having innerconnectedpores of predetermined size, said pores being filled with an atmospherewhich reacts with the molten metal to be cast to produce a condensedsolid oxidized form of said metal which has a low vapor pressure at thecasting temperature, are immersed into the molten metal. Substantiallycomplete filling of said pores with molten metal occurs as a vacuum isproduced by the reaction between the gas in the pores of the mold bodiesand the metal being cast. When the metal to be cast is magnesium, forexample, and the pores of the mold bodies are filled with air, therequired vacuum is generated as the molten magnesium reacts with theoxygen and nitrogen of the air to form magnesium oxide and magnesiumnitride. When the metal is to be cast is aluminum, the pores of the moldbodies may be filled with oxygen. Following the cooling of the metalimpregnated composite bodies to solidify the metal, the cooled compositebodies are ground or crushed to free the metal from the matrix materialand usually also to break or shear the metal which was contained withinthe innerconnecting channels or passages between said pores so as toobtain metal particles of said predetermined desired size. Theparticulate metal is then separated from the residual finely dividedmatrix material and subsequently recovered as herein'before described.

An unexpected advantage of the present method is that it has been foundthat metal impregnated porous 5 media such as, for example, coke, porouscarbon, porous alumina and basic (MgO-SiO brick, are quite friable andthus easily reduced by crushing or grinding to a sufficiently reducedsize to permit ready separation of very finely divided matrix materialfrom the particulate metal by conventional screening. For example,magnesium impregnated metallurgical coke has been hammer-milled so thatsubstantially complete separation of the resulting mixture of pulverizedcoke and magnesium metal particles was achieved, the bulk of the cokepassing through a US. Standard 270 mesh screen and approximately 95percent of the magnesium metal particles being retained on the 270 meshscreen.

Another advantage of the present method is that the particle size of theparticulate metal to be produced can be largely controlled by selectionof a porous matrix or carrier material of the desired porosity. Furtherparticle size control can be achieved by proper selection of the type ofcrushing equipment and conditions employed as is understood by oneskilled in the art, and as hereinbefore described.

Carrier materials suitable for use in the present method are those whichare inert to the metal at process temperatures, are readily crushable,and have melting points higher than that of the metal. Illustrativecommercially available materials are, for example, coke, porous carbon,porous alumina, porous MgO-SiO brick, porous burnt lime and porouszirconia.

Carriers of a desired porosity also can be made expressly for use in themethod of the present invention. For example, briquettes of calcineddolomite can be prepared from a briquetted mixture of MgO-CaO, acarbonaceous fuel and water by removing the water at a temperature ofabout 500 F., burning out the carbonaceous material at a temperature offrom about 1200 to 1800 F., and sintering the briquettes at atemperature of about 2500 F. Metals vacuum cast in such syntheticbriquettes will have a particle size and particle size distributionsubstantially that of the carbonaceous material used in forming thebriquettes.

The present process is suitable for preparing a variety of metals inparticulate form, and is especially suitable for preparing particulatelight metals such as magnesium, aluminum and alloys thereof which arenot readily fabricated in finely divided form.

The production, transfer and other handling of molten metal, thegrinding or crushing of metal impregnated composites, and subsequentseparation and metal recovery operations and the like can be carried outusing conventional techniques and equipment as is well understood by oneskilled in the art.

The following examples will serve to illustrate further the persentinvention but are not meant to limit it thereto.

EXAMPLE 1 A piece of porous aluminum oxide (alumina) weighing 3.69 gramswas preheated in air at about 2000 F. for about one hour. The hotalumina was then immersed into a magnesium melt at about 1400 F. forabout one minute. The piece was withdrawn from the melt and quenched bycovering with a carbonaceous fire extinguishing powder. When theresulting magnesium impreg; nated alumina had cooled, the magnesiumadhering to the surface of the piece was removed. The compositemagnesium impregnated aluminum oxide was weighed and found to contain12.14% magnesium by weight. The magnesium-alumina composite was groundmanually using mortar and pestle. The alumina matrix crumbled easily.The resulting mixture was screened and then found to contain magnesiumparticles ranging in size from about 200 microns to about 50 microns.

EXAMPLE 2 Following the procedure described in Example 1, a porouscarbon matrix having a porosity of about 48%, an apparent average porediameter of 69 microns, and Weighing 10.69 grams was preheated in air atabout 2400 F. and immersed into molten magnesium at about 1380 F. Theresulting magnesium impregnated carbon contained 32.3% magnesium byweight. Sieve analysis following grinding by mortar and pestle showedthe magnesium particles to range in size from about 200 to about 75microns.

EXAMPLE 3 Using the procedure of Example 1, a piece of porous carbonhaving a porosity of 48%, an average pore diameter of 33 microns, andweighing 22.28 grams was preheated at about 2400" F. and immersed intomolten magnesium at about 1380 F. The impregnated composite containing22.9% magnesium by Weight was crushed by mortar and pestle. Themagnesium particles were found to range in size from about to about 50microns by screen analysis.

EXAMPLE 4 Using the same procedure, a piece of metallurgical grade cokeweighing 17.95 grams was immersed into molten magnesium at about 1410"F. for about one minute after preheating at about 1800 F. Themagnesiumcoke composite containing about 34% magnesium by weight wascrushed by means of mortar and pestle. The average magnesium particlesize was estimated to be about 1430 microns by visual observationutilizing a microscope with micrometer eyepiece.

EXAMPLE 5 A piece of commercial basic brick analyzing 71.7% MgO, 19.8%SiO 4.5% A1 0 1.3% CaO and 0.7% Fe O having a porosity of 65%, andweighing 10.29 grams was wired into a graphite cup. The cup was invertedand plunged into molten magnesium at about 1390 F. for about one minutewith no preheating. The resulting magnesium impregnated brick compositewas found to contain 47.2% magnesium by Weight. The average size of themagnesium particles was estimated to be about 600 microns by visualobservation as in Example 4 after crushing the composite by mortar andpestle.

Various modifications can be made in the process of the presentinvention without departing from the spirit or scope thereof, it beingunderstood that we limit ourselves only as defined in the appendedclaims.

We claim:

1. A process for producing. particulate metal which comprises:

(a) forming an integral composite body comprising a solid, crushablematrix structure, said structure defining a multiplicity ofinterconnected pores in which a metal is solidified from liquid formwithin said pores of said matrix structure,

(b) crushing said composite body thereby to free particulate metal fromsaid matrix structure and provide a mixture of a particulate metal andparticulate metal material.

2. The process of claim 1 and including the step of separating theparticulate metal from said mixture of particulate metal and matrixmaterial.

3. A process which comprises the steps of (a) introducing a molten metalinto the pores of a solid, crushable matrix structure, said structurebeing thermally stable at the temperature of said molten metal and notdetrimentally attacked by said metal,

(b) cooling the metal containing matrix structure to solidify the metalwithin said structure thereby to provide said integral composite body,and

(c) crushing said composite body thereby to free particulate metal fromsaid matrix structure and provide a mixture of a particulate metal andparticulate matrix material.

4. The process of claim 3 wherein said molten metal is introduced intosaid pores of said matrix structure by vacuum casting.

5. The process of claim 4 wherein said metal is selected from the groupconsisting of magnesium, magnesium-base alloys, aluminum, aluminum-basealloys, and mixtures thereof.

6. The process of claim 4 wherein said porous matrix structure isaluminum oxide (alumina).

7. The process of claim 4 wherein said porous matrix structure is coke.

8. The process of claim 4 wherein said metal is magnesium and saidporous matrix structure is coke.

References Cited UNITED STATES PATENTS 8/1901 Lovett 241--14 4/1962Justheim et a1 17671 1/ 1963 Dombrowski 24129 L. DEWAYNE RUTLEDGE,Primary Examiner T. W. FRYE, Assistant Examiner US. Cl. X.R.

222 3 UNITED STATES PATENT OFFICE CERTIFICATE OF GORRECTION Patent No-3, 82,96?) aDecember 9. lQYO Invent r( Q l iver Osborn and John C.Robertson It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

In Col. l, line 6, delete "metal" and insert --matrix--.

SIGNED AN'D SEALED 6 Atteat:

Edward M. Fletcher, Ir. Attesting Officer WmIAM E. 'SQHUYLER,Commissioner or Pat-e:

